1. Field of the Invention
The present invention relates to an optical scanner to be used for a laser printer and the like, more particularly to a post-objective type optical scanner in which optical flux is converted to condensed light by a condensing lens and scanned by an optical deflector, and an image forming apparatus using the same.
2. Description of the Prior Art
An optical scanner which is frequently used for the conventional laser printer and the like is a pre-objective type. The pre-objective type optical scanner has a condensing lens arrangement after the optical deflection, and it is easy to compensate the curvature of field and .function. .theta. characteristic by the condensing lens. However, due to the necessity to use large size condensing lens and sophisticated constitution, the optical scanner is expensive and also, due to the necessity to reserve a long optical path, it has difficulty of being formed into small size. Recently, a post-objective type optical scanner in which a condensing lens is disposed before the optical deflection has come to be studied and commercialized as one which can realize a small sized, low priced optical scanner.
As an example of the conventional post-objective type optical scanner, there is one as disclosed in Japanese Patent Laid-open Publication No. 2-169422/1989. In this prior art, the curvature of field in the scanning direction is compensated by making the reflecting surface of the optical deflector cylindrical or spherical, and the curvature of field in the sub-scanning direction is compensated by varying the power of the compensating lens disposed between the optical deflector and the scanning surface from the center to the periphery in the scanning direction. However, the system has a problem of difficulty to make the size small because of the arrangement of a lens between the optical deflector and the scanning surface.